A technique is known, in which a selective catalytic reduction NOx catalyst (hereinafter referred to as “SCR catalyst” as well) is arranged at an exhaust gas passage of an internal combustion engine. The SCR catalyst can adsorb NH3 and reduce NOx by using NH3.
In this context, a technique is known, in which a urea addition valve, an SCR catalyst, an ammonia oxidation catalyst, and a NOx sensor are provided successively in this order from the upstream side at an exhaust gas passage of an internal combustion engine to judge whether or not any abnormality arises in each of the SCR catalyst, the ammonia oxidation catalyst, the urea addition valve, and the NOx sensor (see, for example, Patent Document 1).
Further, a technique is known, in which the deterioration judgment is carried out for an SCR catalyst by using an NH3 sensor (see, for example, Patent Document 2).
In the meantime, it is demanded to further reduce the amount of emission (discharge amount) of NH3 to be emitted or discharged into the atmospheric air. In relation thereto, it is conceived that the supply amount of the reducing agent is controlled on the basis of the detection value of the NH3 sensor. However, any abnormality may possibly arise in the NH3 sensor. For this reason, it is also necessary to judge whether or not the NH3 sensor is abnormal. The abnormality judgment for the NH3 sensor has not been investigated so much until now, and hence there is room for investigation.